Continuity mistake: When Alison gets the gun from Zep and aims it at him, the gun is in her left hand. In the next shot it's in her right hand, and then it moves back to her left hand again. This is not an artifact of changing camera angles, you can clearly see the different hands holding the gun. (01:20:30)
Continuity mistake: When Larry is desperately trying to reach for the cell phone, he uses the box that it came in. When that doesn't work he throws it off to the side (his left) but when we see it again the box is back where it originally was (on his right).
Continuity mistake: When Dr Gordon begins to saw his foot off, the blood seen covering his neck and top disappear in subsequent shots. (01:26:40)
Continuity mistake: In the scene where. Lawrence and Adam find the 'X' on the wall, there is a continuity error. Right after they find the 'X', the lights are turned on and there is the shape of a square on the wall (the cavity where the box is hidden). Before they find the 'X' there is no box or any dirty lines where the cavity is. (00:54:00)
Continuity mistake: When Dr. Lawrence opened his envelope he got a key in it. When he tried it on the lock on his chain and it failed to open, Adam then asks to try it. Right when Adam asks for it, he puts his tape and envelope down separate, but when reaches for his tape again, it has ended up inside the envelope. (00:07:50)
Continuity mistake: When Dr. Gordon gets out of Tapp's car and is talking to him at the car window, the amount of shirt sleeve showing under his jacket changes between shots. (00:42:00)
Continuity mistake: When Dr Gordon is going crazy, he throws the box to his right, but in the next shot it's on the left, then on the right again. (01:25:50)
Continuity mistake: When Adam retrieves the bag with the saws from the toilet, he places it on his right side. At one point you can see that there is nothing on his left side, but the next shot shows Adam grabbing the bag from his left side and tossing it into the tub. (00:14:00)
Continuity mistake: When Adam is walking into his apartment, his camera has a short lens on it. He then rounds the corner and goes into the darkroom and the camera has a long lens on it.
Continuity mistake: When Adam throws the picture that he found in Lawrence's wallet to him, the picture lands next to the rectangular piece of tile that Lawrence uses in the immediate next shot to bring the picture closer to him. The tile moves from in front of him (given it's proximity to the photo) to right beside his right arm/hand. It "moved" about two feet.